Method of Producing a Polypeptide or Virus of Interest in a Continuous Cell Culture

ABSTRACT

Described herein is a chemostat-like continuous cell culture system that combines certain advantages of perfusion open systems and chemostat open systems to improve the culturing of mammalian cells, e.g., genetically modified cells, particularly in serum-free or chemically-defined media. The continuous culture system described herein involves culturing mammalian cells in a continuous cell culture system, which comprises a cell retention device, wherein the cell culture system has a dilution rate (D) of less than about 2 d −1 , and a cell density of less than about 2×10 7  cell/mL. Also described herein is a method for producing a polypeptide and/or virus of interest in a continuous cell culture, the method comprising culturing mammalian cells expressing the polypeptide and/or virus of interest in a continuous cell culture system, which comprises a cell retention device, wherein the cell culture system has a dilution rate (D) of less than about 2 d −1 , and a cell density of less than about 2×10 7  cell/mL; and recovering the polypeptide and/or virus of interest from medium of the cell culture system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.61/230,313, filed Jul. 31, 2009, which is hereby incorporated byreference in its entirety, and to which application we claim priority.

FIELD OF THE INVENTION

The present invention relates to a continuous cell culture strategy forproducing polypeptides or viruses of interest in mammalian cell culture.The continuous cell culture strategy described herein combines theadvantages of chemostat and perfusion culture technologies.

BACKGROUND OF THE INVENTION

The ability to produce a polypeptide or virus of interest isincreasingly important to the biotechnology industry. Over the last twodecades, advances in biotechnology have led to interest in numerouspolypeptides and viruses that have potential therapeutic uses asvaccines and pharmaceuticals. Large scale production generally hasinvolved recombinant production of the polypeptide or virus of interest,e.g., in bacterial, yeast, insect, mammalian, or other cell types. Theproduction of polypeptides or viruses of interest in mammalian cultures,in particular, has advantages over production in bacterial or otherlower microbial hosts because of the ability of mammalian cells topost-translationally process complex protein structures, via, e.g,disulfide-dependent folding and glycosylation.

Mammalian cells are generally propagated in vitro in one of two modes:as non-anchorage dependent cells growing freely in suspension throughoutthe bulk of the culture; or as anchorage-dependent cells requiringattachment to a solid substrate for their propagation (i.e., a monolayertype of cell growth). Microcarrier systems have been developed toaccommodate both types of growth. For example, anchorage-dependent cellsmay be propagated in microcarrier systems comprising small solidparticles suspended in growth medium by slow agitation. This systemallows anchorage-dependent cells to attach to the surfaces of thesuspended particles, and grow to confluency, while the microcarriersremain suspended in the growth medium. Alternatively, macroporousmicrocarriers may be used to contain non-anchorage dependent cells inbioreactors, e.g., by nonspecific attachment for the cells to thesurface of such microcarriers. Microcarrier suspension cultures ofeither non-anchorage dependent cells or anchorage-dependent cells arethe most widely used means of large scale production of cells and cellproducts.

Large-scale suspension cultures may be operated in a closed system, forexample, as batch or fed-batch closed systems, which are morestraightforward to operate and scale up than open systems. Typically, ina closed system, no cells, products, and/or waste products are removed(although air (e.g., oxygen) may be added and CO₂ removed by aeration).The typical growth profile seen with batch growth systems involves a lagphase, followed by an exponential phase, a stationary phase, and adecline phase. In such batch systems, the environment is continuouslychanging, as nutrients are depleted and metabolites accumulate. In a fedbatch system, key nutrients are continuously fed into the system toprolong the growth cycle although cells, products, by products, andwaste products, including toxic metabolites, are not removed.Accordingly, production of a polypeptide or virus of interest by thebatch or fed batch systems is limited by the accumulation of cells andharmful substances, such as toxic metabolites.

Large-scale suspension cultures may also be operated in an open system,e.g., a perfusion system or a chemostat system. In a perfusion system,fresh medium is perfused through the culture while the cells areretained with a variety of cell retention devices. Types of cellretention devices include, for example, microcarriers, fine mesh spinfilters, hollow fibers, flat plate membrane filters, settling tubes,ultrasonic cell retention devices, and the like. Typically, perfusioncultures are designed to increase cell densities to a maximum, with cellretention devices designed and operated to have a cell retention rateof >90%. Such cultures typically reach cell densities of >2×10⁷, whichmay have to be supplied with a cell culture medium feed at a dilutionrate greater than about 2.0 d⁻¹. However, the steady state of the systemis hard to maintain because of the uncontrolled increase in biomass, andconsistent production conditions are difficult to control and/orachieve.

Chemostat systems are operated with a continuous inflow of medium and anoutflow of cells and products. In the chemostat system, there is no cellretention device, such that the concentration of cells in the bioreactorand the concentration of the cells in supernatant harvested from thebioreactor are substantially identical. Typically, culture medium is fedto the reactor at a predetermined and constant rate, maintaining a lowdilution rate of the culture (typically 0.3 d⁻¹ to 0.8 d⁻¹). To preventwashout of cells, the dilution rate generally is chosen to be less than,and sometimes equal to, the maximum specific growth rate of the cells.Culture fluids containing cells, cell products, byproducts, wasteproducts, etc., are removed at the same rate, or substantially the samerate. Chemostat systems typically provide for a high degree of control,since the cultures may equilibrate, i.e., reach a steady state at aspecific growth rate equivalent to the dilution rate. This equilibrationis determinative of the concentration of the cells, metabolites, wasteproducts, expressed products (e.g. secreted proteins), etc. Specificgrowth rates in chemostat systems are typically lower than the maximumgrowth rate due to at least one limiting substrate. In some systems,however, steady states may be maintained at the maximum specific growthrates by controlling and adjusting biomass, e.g., in turbidstat systemsof chemostat cultures. Preferably, such chemostat cultures contain ahomogeneous distribution of cells (e.g., single cell suspensions)throughout the bioreactor. Compared to the perfusion system, however,the chemostat system typically results in lower cell densities.Furthermore, an inherent disadvantage of chemostat systems is that thefeed of the cells can not be controlled independent of the celldensities in the bioreactor system.

Suspension cell cultures for producing recombinant proteins inserum-free and/or chemically-defined media are also limited in that theserum-free and/or chemically-defined media typically support slowergrowth rates compared to cells grown in media containing serum. Loweredgrowth rates in the culture means a lowered production of thepolypeptide or virus of interest.

Accordingly, there remains a need for the development of cell culturesystems capable of sustaining production of a polypeptide or virus ofinterest, especially for cultures that can be sustained for prolongedperiods of time, for example, to meet demands for increased productionat low costs. The present invention provides methods and compositionsdirected at meeting this and other needs.

SUMMARY

Disclosed herein is a continuous cell culture for the production ofpolypeptides and/or viruses of interest in mammalian cells, particularlynon-anchorage dependent cells. The continuous cell culture methoddisclosed herein combines advantages of perfusion open systems andchemostat open systems and allows for increased cell density and cellgrowth, particularly in serum-free or chemically-defined media. Theincreased cell density and cell growth provides for improved yields ofproteins and/or viruses of interest while allowing more control overprocess parameters.

Accordingly, one aspect of the invention relates to a method forproducing a polypeptide and/or virus of interest in a continuous cellculture, the method comprising culturing mammalian cells expressing thepolypeptide and/or virus in a continuous cell culture system, whereinthe cell culture system comprises a cell retention device and has adilution rate of less than about 2 d⁻¹ and a cell density of less thanabout 2×10⁷ cell/mL; and recovering the polypeptide and/or virus ofinterest from medium removed from the cell culture system. In someembodiments, the cells are genetically modified to recombinantly expressthe polypeptide and/or virus of interest.

In some embodiments, the cell retention device is chosen to be lesscapable of retaining cells than typical cell retention devices. In someembodiments, the cell retention device produces a cell retention rate ofless than about 90%, less than about 85%, less than about 80%, or lessthan about 75%. In some embodiments, the cell retention device comprisesa macroporous microcarrier, e.g., a cellulose-based particle.

The dilution rate and cell density preferably are maintained at chosenvalues or ranges. In some embodiments, the dilution rate is less thanabout 1 d⁻¹, e.g., between about 0.1 and about 1.0 d⁻¹. In someembodiments, the cell density is less than about 1×10⁷ cell/mL. In someembodiments, the ratio of the dilution rate and the specific growth rate(D/μ) of the cell culture system is maintained at a chosen value orrange. In some preferred embodiments, the cell culture system has aratio of the dilution rate and specific growth rate greater than about1, for example, between about 1.2 and about 5.0, or between about 1.8and about 3. In some embodiments, the specific growth rate is maintainedbetween 0.2 and 0.8 d⁻¹.

An advantage of certain embodiments of the continuous cell culturesystem described herein is that the culture can be sustained for aprolonged period of time. In some embodiments, for example, the dilutionrate and/or the cell density are maintained for at least about 80% ofthe time the cells are being cultured in the continuous cell culturesystem. In some embodiments, the cells are cultured in the cell culturesystem for more than 20 days, preferably more than 40 days, and morepreferably, more than 50 days, for example, allowing increasedproduction of the polypeptide and/or virus of interest, potentially atlow cost.

Another advantage of certain embodiments of the continuous cell culturesystem is an increase in volumetric productivity due to higher celldensity, compared to a typical chemostat culture. For example, inparticular preferred embodiments, volumetric productivity increases by70%, 90%, or more. Still another advantage of certain embodiments of thecontinuous cell culture system is improved specific activity of theprotein being recovered, due to, for example, reduced residence time inthe culture unit, which may have beneficial effect on stability,structure, and/or function of the protein.

Another advantage of certain embodiments of the continuous cell culturesystem described herein is that the system is amenable to being scaledup for large scale productions. That is, by allowing increased celldensity and/or cell growth, as compared to standard chemostat culturesystems, the continuous cell culture system disclosed herein providesfor commercial scale production of a polypeptide and/or virus ofinterest. For example, in some embodiments, the cells are cultured in atleast about 250 L of medium, e.g., at least abut 500 L, or at leastabout 1,000 L of medium. In some preferred embodiments, the cells arecultured in serum-free media and/or chemically-defined media.

In some embodiments, the method further comprises a pre-culturing step,e.g., to adapt the cells for production of the polypeptide and/or virusof interest in a continuous cell culture system as herein described.Thus, in some preferred embodiments, the method further comprises,before the culturing step, pre-culturing the cells in suspension, e.g.,until the culture reaches an appropriate volume.

In a particular embodiment, the polypeptide of interest is adisintegrin-like and metallopeptidase with thrombospondin type 1 motif13 (ADAMTS13) protein. In some embodiments, the mammalian cells are CHOcells, e.g., CHO cells genetically modified to express ADAMTS13 protein.In a particular embodiment, a method is provided for producing apolypeptide and/or virus of interest in a continuous cell culture, themethod comprising (a) culturing non-anchorage dependent mammalian cellsexpressing the polypeptide and/or virus of interest in a continuous cellculture system, where the cell culture system comprises a cell retentiondevice having a cell retention rate of less than 90%, and has a dilutionrate (D) between 0.1 and 1.0 d⁻¹ and a cell density of less than 1×10⁷cell/mL; and (b) recovering the polypeptide and/or virus of interestfrom medium removed from the cell culture system.

In another particular embodiment, a method is provided for producing apolypeptide and/or virus of interest in a continuous cell culture, themethod comprising (a) culturing non-anchorage dependent cells CHO cellsexpressing the polypeptide and/or virus of interest in a continuous cellculture system, where the cell culture system comprises a macroporousmicrocarrier cell retention device having a cell retention rate of lessthan 90%, and has a dilution rate (D) between 0.1 and 1.0 d⁻¹ and a celldensity of less than 1×10⁷ cell/mL; and (b) recovering the polypeptideand/or virus of interest from medium removed from the cell culturesystem.

In still another particular embodiment, a method is provided forproducing ADAMTS13 in a continuous cell culture, the method comprising(a) culturing non-anchorage dependent cells mammalian cells expressingrecombinant ADAMTS13 protein in a continuous cell culture system, wherethe cell culture system comprises a macroporous microcarrier cellretention device having a cell retention rate of less than 90%, and hasa dilution rate (D) between 0.1 and 1.0 d⁻¹ and a cell density of lessthan 1×10⁷ cell/mL; and (b) recovering the ADAMTS13 from medium removedfrom the cell culture system.

In yet another particular embodiment, a method is provided for producinga polypeptide and/or virus of interest in a continuous cell culture, themethod comprising (a) culturing non-anchorage dependent cells mammaliancells expressing the polypeptide and/or virus of interest in acontinuous cell culture system, where the cell culture system comprisesa cell retention device having a cell retention rate of less than 90%,and has a dilution rate (D) between 0.1 and 1.0 d⁻¹, a cell density ofless than 1×10⁷ cell/mL, and a ratio of the dilution rate and specificgrowth rate (D/u) between 1.2 and 5; and (b) recovering the polypeptideand/or virus of interest from medium removed from the cell culturesystem; and further where the cells are cultured in the cell culturesystem for more than 50 days, and the dilution rate, cell density, andratio of the dilution rate and specific growth rate each are maintainedfor at least 80% of the time the cells are cultured in the cell culturesystem.

A further aspect of the invention relates to a composition comprising apolypeptide and/or virus of interest produced according to methodsdescribed herein, e.g., a composition comprising a recombinant ADAMTS13protein produced accordingly.

These and other aspects of the invention are described in more detailbelow.

DETAILED DESCRIPTION

The continuous cell culture system as described herein combines some ofthe advantages of both perfusion culture and chemostat culture ofmammalian cells. As described above, perfusion culture systems areoperated with fresh medium perfusing through the cell culture, while thecells are retained using a cell retention device; chemostat culturesystems are operated with a continuous inflow of media and outflow ofcells and products, without a cell retention device.

As used herein “perfusion” refers to continuous flow of a physiologicalnutrient solution at a steady rate, through or over a population ofcells. As perfusion systems generally involve the retention of the cellswithin the culture unit, perfusion cultures characteristically haverelatively high cell densities, but the culture conditions are difficultto maintain and control. In addition, since the cells are grown to andthen retained within the culture unit at high densities, the growth ratetypically continuously decreases over time, leading to the lateexponential or even stationary phase of cell growth. In contrast,“chemostat” as used herein, refers to continuous inflow of aphysiological nutrient solution coupled with a continuous outflow ofcells and other products with drawn media, through a cell culture, e.g.,as in chemostat systems. However, because cells are continuouslyremoved, the chemostat system typically supports only lower celldensities.

The continuous culture strategy described herein generally comprisesculturing mammalian cells, e.g., non-anchorage dependent cells,expressing a polypeptide and/or virus of interest during a productionphase in a continuous cell culture system. By “non-anchorage dependentcells” is meant cells propagating freely in suspension throughout thebulk of a culture, as opposed to being attached or fixed to a solidsubstrate during propagation. The continuous cell culture system willcomprise a cell retention device similar to that used in a perfusionsystem, but that allows continuous removal of a significant portion ofthe cells, preferably such that a smaller percentage of the cells areretained than in perfusion culture. By “cell retention device” is meantany structure capable of retaining cells, particularly non-anchoragedependent cells, in a particular location during cell culture.Nonlimitng examples include microcarriers, fine mesh spin filters,hollow fibers, flat plate membrane filters, settling tubes, ultrasoniccell retention devices, and the like, that can retain non-anchoragedependent cells within bioreactors. Polypeptides and/or viruses ofinterest (e.g., a recombinant polypeptide and/or recombinant virus) canbe recovered from the cell culture system, e.g., from medium removedfrom the cell culture system.

The method for producing a polypeptide and/or virus of interest (e.g., arecombinant polypeptide and/or recombinant virus) disclosed hereincomprises a cell culture method that provides a chemostat-like culturesystem and that uses a cell retention device. The system comprisesculturing mammalian cells expressing a polypeptide and/or virus ofinterest in a continuous cell culture system with a cell retentiondevice. The role of the cell retention device is to prevent the removalfrom culture a portion, preferably a substantial portion, of the viablecells during replenishment of the spent culture medium with freshmedium. A successful cell retention device should fulfill as many aspossible of the following requirements: (1) minimal cell damage oreffect on cell growth and productivity, (2) selective retention ofviable cells only (nonviable cells preferably are removed from theculture since they release toxic metabolites into the cultureenvironment), (3) uninterrupted operation for long periods ofcultivation, (4) low energy consumption, (5) simplicity in operation andmaintenance, (6) scale-up capabilities for large scale production units,(7) compact structure, and (8) cost effectiveness.

In particularly preferred embodiments, the cell retention device usedpermits partial retention of the cells. Cell retention devices and/ormethods are well-known in the art. Many are based on conventionalsedimentation, centrifugation, and/or filtration techniques. Nonlimitingexamples of cell retention devices include microcarriers, spin filters,such as fine mesh spin filters, hollow fibers, flat plate membranefilters, settling tubes, ultrasonic cell retention devices, gravitysettlers, centrifuges, acoustic cell filters, dielectrophoresis-basedcell separators, and the like (see, e.g., U.S. Pat. Nos. 5,019,512;5,626,734, which hereby are incorporated by reference in its entirety).

According to one embodiment of the culture system described herein,microcarriers may be used as the cell retention device. The microcarriermay act as a low cost scalable surface on which non-anchorage dependentcells may be immobilized in order to aid cell retention. As used herein,“microcarriers” refer to particles small enough to be used in suspensioncultures, preferably with a stirring rate that does not causesignificant shear damage to cells. Microcarriers may be solid, porous,or have a solid core with a porous coating. Microcarriers may, forexample, without limitation, be cellulose- or dextran-based, and theirsurfaces (exterior and interior surface in case of porous carriers) maybe positively charged. Further details regarding microcarriers can befound, e.g., in WO 02/29083, which hereby is incorporated by referencein its entirety.

In one embodiment, the microcarrier is a macroporous microcarrier. Asused herein, “macroporous microcarriers” refers to particles, e.g.cellulose-based particles, which have the following properties: (a) theyare small enough to allow use in suspension cultures, preferably with astirring rate that does not cause significant shear damage to cells; and(b) they have pores and interior spaces of sufficient size to allowcells to migrate into the interior spaces. Their surfaces (exterior andinterior) may in one embodiment be positively charged. In oneembodiment, the carriers: (a) have an overall particle diameter betweenabout 150 and about 350 (b) have pores having an average pore openingdiameter of between about 15 and about 40 μm; and (c) have a positivecharge density of between about 0.8 and 2.0 meq/g. In some embodiments,the positive charge is provided by DEAE (N,N,-diethylaminoethyl) groups.Useful macroporous microcarriers include, without limitation, CYTOPORE1™ and CYTOPORE 2™ (GE Healthcare Life Sciences, Piscataway, N.J.).Particularly useful macroporous microcarriers are CYTOPORE 2™ carriers,which have a mean particle diameter of 230 μm, an average pore size of30 μm, and a positive charge density of 1.8 meq/g.

In some embodiments, the culture units are agitated. Agitation maycomprise shaking, stirring, rocking, vibrating, or the like, as known inthe art. Lou preferred embodiment, agitation is achieved with a Rushtontype impeller with baffles. The baffles may be at about 140 rpm, whichcorresponds to a specific power/volume input of approximately 40 W/m³.In some embodiments, the specific power/volume input is more than about40 W/m³, e.g., about 50 W/m³, about 60 W/m³, about 70 W/m³, about 80W/m³ or more. Other speeds may be used as suitable according to theparticular cells or culture being used.

The concentration of the microcarriers as described herein is generallylow, for example, to aid in maintaining dilution rate and cell densityin specific ranges. In one embodiment, the culture unit may comprise anamount of microcarriers corresponding to a final microcarrierconcentration in the range of 0.05-1.0 g/L. In one embodiment, the finalmicrocarrier concentration is about 0.05-0.1 g/L. In one embodiment, thefinal microcarrier concentration is about 0.1-0.25 g/L. In anotherembodiment, the final microcarrier concentration is about 0.25-0.5 g/L.In another embodiment, the final microcarrier concentration is about0.5-0.75 g/L. In another embodiment, the final microcarrierconcentration is about 0.75-1.0 g/L. In another embodiment the carrierconcentration may be increased or reduced during the continuous culture,e.g., to adjust cell densities and dilution rates within predeterminedranges.

The disclosed continuous cell culture system has a preferred dilutionrate (D) and a preferred cell density. In particular, the dilution rateand cell density are maintained at predetermined values or withinpredetermined ranges. Furthermore the disclosed continuous cell culturesystem may have a minimum specific growth rate or a predetermined rangeof specific growth rates over the entire process time.

Dilution rate (D) refers to the volume of medium supplied per daydivided by the volume of the culture. Although the continuous cellculture system described herein involves cell retention, the dilutionrate of the continuous cell culture system is generally less than thatof a perfusion culture, e.g., less than about 2 dilution volume/day (2d⁻¹). In one embodiment, the dilution rate is maintained between morethan about 0.2 d⁻¹ to less than about 2.0 d⁻¹. In another embodiment,the dilution rate is maintained at less than about 2.0 d⁻¹, e.g., lessthan about 1.8 d⁻¹, e.g., less than about 1.5 d⁻¹, e.g., less than about1.2 d⁻¹, etc. In another embodiment, the dilution rate is maintained atless than about 1.0 d⁻¹,e.g., less than about 0.9 d⁻¹, e.g., less thanabout 0.8 d⁻¹, e.g., less than about 0.7 d⁻¹, e.g., less than about 0.6d⁻¹, etc. In another embodiment, the dilution rate is maintained at morethan about 0.2 d⁻¹, e.g., more than about 0.3 d⁻¹, e.g., more than about0.4 d⁻¹, e.g., more than about 0.5 d⁻¹, etc.

Additionally, in a continuous cell culture system as described herein,the cell density is maintained at a value less than that maintained in aperfusion culture system but higher than cell densities achieved in achemostat system. In one embodiment, the cell density is less than about2×10⁷ cell/mL, e.g., less than about 1.5×10⁷ cell/mL, e.g., less thanabout 1×10⁷ cell/mL, e.g., less than about 8×10⁶ cell/mL, e.g. less thanabout 6×10⁶ cell/mL, e.g., less than about 5×10⁶ cell/mL. In anotherembodiment, the cell density may be more than about 1×10⁶ cell/mL, e.g.,more than about 1.5×10⁶ cell/mL, e.g. more than about 2×10⁶ cell/mL,e.g. more than about 3×10⁶ cell/mL, e.g. more than about 4×10⁶ cell/mL,etc. In another embodiment, the cell density is maintained at betweenabout 1.0×10⁶ cell/mL to about 2×10⁶ cell/mL. In another embodiment, thecell density is maintained at between about 2×10⁶ cell/mL to about 4×10⁶cell/mL. In another embodiment, the cell density is maintained betweenabout 5×10⁶ cell/mL to about 1×10⁷ cell/mL, e.g., between about 6×10⁶cell/mL to about 8×10⁶ cell/mL. In another embodiment, the cell densityis maintained between about 1×10⁷ cell/mL to about 2×10⁷ cell/mL.

A skilled artisan will recognize that a mechanism by which celldensities may be maintained involves decreasing the portion of cellsretained with the cell retention device, i.e., the cell retention rate.Generally, a perfusion culture has a cell retention rate of greater than90% or 95%, and, in many cases, close to 100%. In the disclosedcontinuous cell culture system, the cell retention rate is less than90%. In one embodiment, the cell retention is less than about 85%, e.g.,less than about 75%. In one embodiment, the cell retention rate is lessthan about 70%, e.g., less than about 60%, e.g., less than about 50%,e.g., less than about 40%, e.g., less than about 30%. In one embodiment,cell retention is maintained between about 30% and about 90%. In anotherembodiment, cell retention is maintained between about 30% and about80%. In another embodiment, cell retention is maintained between about30% and about 70%. In another embodiment, cell retention is maintainedbetween about 40% and about 60%. In another embodiment, cell retentionis maintained between about 40% and about 70%. In another embodiment,cell retention is maintained between about 50% and about 90%. In anotherembodiment, cell retention is maintained between about 60% and about90%. In another embodiment, cell retention is maintained between about70% and about 90%. In another embodiment, cell retention is maintainedbetween about 80% and about 90%.

The culture may also be characterized by the ratio of the dilution rateand the specific growth rate. Specific growth rate (μ) refers to theincrease in cell mass per cell mass per day (relative to the total cellmass): μ=(ln(X_(t)/X_(t-1)))/((t)−(t−1)); wherein X_(t) is the biomassconcentration at time (t) and X_(t-1) is the biomass at previoustimepoint. Generally, the specific growth rate of the continuous culturesystem as described herein should be constant within a predeterminedrange, and preferably with a certain minimum level to ensure a minimumof a growth-associated polypeptide and/or virus expression. For example,the specific growth rate of the continuous culture system describedherein may be from about 0.1 d⁻¹ to about 1.0 d⁻¹. In one embodiment,the specific growth rate maintained by the continuous culture systemdescribed herein is greater than about 0.1 d⁻¹, e.g., greater than about0.15 d⁻¹, e.g., greater than about 0.2 d⁻¹, e.g., greater than about0.25 d⁻¹, e.g., greater than about 0.3 d⁻¹, e.g., greater than about0.35 d⁻¹, etc. In another embodiment, the specific growth ratemaintained by the continuous culture system described herein is lessthan about 1.0 d⁻¹, e.g., less than about 0.8 d⁻¹, e.g., less than about0.7 d⁻¹, e.g., less than about 0.6 d⁻¹, e.g., less than about 0.5 d⁻¹,e.g., less than about 0.45 d⁻¹. In another embodiment, the specificgrowth rate may be maintained between about 0.1 d⁻¹ to about 0.45 d⁻¹.In one embodiment, the specific growth rate maintained by the continuousculture system described herein is between about 0.15 d⁻¹ to about 0.3d⁻¹. In one embodiment, the specific growth rate maintained by thecontinuous culture system described herein is between about 0.2 d⁻¹ toabout 0.25 d⁻¹.

As described above, the continuous culture system disclosed hereinmaintains a cell density less than about 2×10⁷ cell/mL. A mechanism bywhich the cell densities may be maintained as described above involvesmaintaining a particular dilution rate to specific growth rate ratio.Chemostat cultures have dilution rates approximately equal to specificgrowth rates for a D/μ ratio of approximately 1. Perfusion culturesgenerally have higher absolute dilution rates and very low specificgrowth rates, so the D/μ ratio is significantly greater than 1. In thecontinuous culture system disclosed herein, however, a dilution ratethat is slightly higher than specific growth rate preferably ismaintained. Accordingly, in the continuous culture disclosed herein, aD/μ ratio greater than 1 is maintained. In particularly preferredembodiments, the dilution rate is calculated and set in accordance withthe efficiency of the retention device so as to maintain the specificgrowth rate within a predetermined range. In one embodiment, the ratioof the dilution rate to the specific growth rate is greater than about1.0, e.g., greater than about 1.2, e.g., greater than about 1.5, e.g.,greater than about 2, e.g., greater than about 2.5. In anotherembodiment, the ratio of the dilution rate to the specific growth rateis less than about 5, e.g., less than about 4, e.g., less than about 3.In one embodiment, the ratio of D/μ is between about 1.2 and about 5. Inanother embodiment, the ratio of the dilution rate to the specificgrowth rate is between about 1.8 and about 3. In another embodiment, theratio of the dilution rate to the specific growth rate is between about2.0 and about 2.5.

The methods of the invention may be carried out in an appropriateculture unit or bioreactor. The bioreactor can be of any size as long asit is useful for culturing cells, e.g., mammalian cells. Since processeswith low cell densities are generally easy to scale up, the methods ofthe invention may be particularly advantageous for large scale culturing(i.e., with culture volumes greater than 250 L) and may be particularlyamenable to scaling up from small, laboratory scale cultures (e.g., 10L) to production scale cultures (e.g., 250 L and greater) with minimalmodification of culture conditions. The internal conditions of theculture unit, including but not limited to pH, pO₂, and temperature, aretypically controlled during the culturing period. A production cultureunit refers to the final culture unit used in the production of thepolypeptide, virus, and/or any other product of interest. The volume ofa large-scale production culture unit is generally greater than about250 liters, and may be about 300, about 500, about 800, about 1000,about 2500, about 5000, about 8000, about 10,000, about 12,0000 L ormore, or any intermediate volume. A suitable culture unit or productionculture unit may be composed of (i.e., constructed of) any material thatis suitable for holding cell cultures suspended in media under theculture conditions contemplated herein, and one that is conducive tomammalian cell growth and viability. Examples of suitable materialsinclude, without limitation, glass, plastic, and/or metal. In preferredembodiments, the material(s) do not interfere, or do not significantlyor do not substantially interfere, with expression and/or stability ofthe desired product, e.g., the polypeptide and/or virus of interest. Oneof skill in the art will be aware of, and will be able to choose,suitable culture units for use in practicing the present continuousculture system.

In some embodiments, the cell culture process is operated in more thanone distinct culture units, such as using one or more seed (propagation)culture unit(s) followed by use of the production culture unit. In someembodiments, then, the process involves transferring about 50 L of thepropagated seed culture (having about 1.0×10⁶ cells/mL) into a 250 Lculture unit containing about 150 L of culture medium. Generally, thecontinuous culture system described herein is applied only to productionculture units. For example, seed mammalian cells may first bepropagated, e.g., in batch, fed-batch, perfusion, and/or chemostatsystems, in one or more seed culture units. After transfer of the cellsto a production culture unit, the cells may be cultured according to thecontinuous culture system as described herein, e.g., with a cellretention device in a cell culture system having a dilution rate of lessthan about 2 d⁻¹ and a cell density of less than about 2×10⁷ cell/mL.

Alternatively, expansion of the cells to the production culture unit andthe production phase may be accomplished in one physical culture unit.For example, the cells may be expanded to a final production scale andthe process switched to production conditions, whereupon the conditionsfor the continuous cell culture system described herein may be used.

It was unexpectedly discovered that the culture methods of the inventionmay be used to maintain cell densities and dilution rate withinpredetermined ranges, for example, to support production phases for aduration similar to that of chemostat or perfusion systems. By“predetermined ranges” is meant target ranges, e.g., ranges describedherein for operation of the continuous culture systems according toembodiments of the instant invention. For example, in some embodiments,the target range for dilution rate is between 0.2 d⁻¹ to 2.0 d⁻¹ and thecell density is less than 2×10⁷ cell/mL. In another embodiment, thedilution rate is maintained at less than 2.0 d⁻¹, e.g., less than 1.8d⁻¹, e.g., less than 1.5 d⁻¹, or e.g., less than 1.2 d⁻¹, and the celldensity is less than 1.5×10⁷ cell/mL, e.g., less than 1×10⁷ cell/mL, ore.g., less than 8×10⁶ cell/mL. In some embodiments, the specific growthrate is 0.1 d⁻¹ to 1.0 d⁻¹ and the ratio of dilution rate to specificgrowth rate is between 1.2 and 5. In some embodiments, the target rangefor dilution rate is between 0.5 d⁻¹ to 1.0 d⁻¹ and the cell density isless than 5×10⁶ cell/mL. In some embodiments, the specific growth rateis 0.15 d⁻¹ to 0.3 d⁻¹ and the ratio of dilution rate to specific growthrate is between 1.8 and 3. In some embodiments, the specific growth rateis 0.2 d⁻¹ to 0.25 d⁻¹ and the ratio of dilution rate to specific growthrate is between 2.0 and 2.5. In a particularly preferred embodiment,cell density is 5×10⁶ cell/mL or less, dilution rate is less than 0.6d⁻¹, and specific growth rate is 0.18 to 0.27 d⁻¹. In anotherparticularly preferred embodiment, cell density is less than 5×10⁶cell/mL, dilution rate is between 0.55 and 0.6 d⁻¹, and specific growthrate is 0.16 to 0.26 d⁻¹. Embodiments include maintaining the dilutionrate and/or cell density and/or specific growth rate and/or ratio ofdilution rate to specific growth rate within a target range (e.g., thoseset forth above) for at least about 50%, e.g. at least about 60%, e.g.,at least about 70%, e.g., at least about 80%, e.g., at least about 90%,e.g., at least about 95%, e.g., at least about 98% of the totalcontinuous culture time and/or production phase time.

The continuous cell culture system described herein can allow sustainedproduction of a polypeptide and/or virus of interest from mammaliancells. In some embodiments, the cells are cultured for a totalcontinuous cell culture time of more than about 7 days. In morepreferred embodiments, the cells are cultured for more than about 9days, more than about 14 days, more than about 21 days, more than about28 days, more than about 35 days, more than about 40 days, more thanabout 45 days, or more than about 50 days.

The terms “cell culture medium” and “culture medium” (or simply“medium”) refer to a nutrient solution used for growing eukaryote cellsthat typically provides at least one component from one or more of thefollowing categories: (1) salts (e.g., sodium, potassium, magnesium,calcium, etc.) contributing to the osmolality of the medium; (2) anenergy source, usually in the form of a carbohydrate such as glucose;(3) all essential amino acids, and usually the basic set of twenty aminoacids; (4) vitamins and/or other organic compounds required at lowconcentrations; and (5) trace elements, where trace elements are definedas inorganic compounds that are typically required at very lowconcentrations, usually in the micromolar range. The nutrient solutionmay optionally be supplemented with one or more of the components fromany of the following categories: (a) animal serum; (b) hormones andother growth factors such as, for example, insulin, transferrin, andepidermal growth factor; and (c) hydrolysates of plant, yeast, and/ortissues, including protein hydrolysates thereof.

The present continuous culture system finds particular use whencultivating mammalian cells expressing a polypeptide and/or virus ofinterest in serum-free medium, chemically-defined medium, or mediumlacking animal-derived components. Chemically defined media are media inwhich all components have a known chemical structure. Chemically-definedmedium are available from commercial suppliers, such as, for example,Sigma, JRH Biosciences, Gibco and Gemini. In other embodiments of theinvention, the medium may contain an amino acid(s) derived from anysource or method known in the art, including, but not limited to, anamino acid(s) derived from adding one or more amino acids or adding apeptone or protein hydrolysate (including hydrolysate from an animal,yeast, or plant source(s)).

Any cell culture medium that supports cell growth and maintenance underthe conditions of the invention may be used. Typically, the mediumcontains water, an osmolality regulator, a buffer, an energy source,amino acids, an inorganic or recombinant iron source, one or moresynthetic or recombinant growth factors, vitamins, and cofactors. Inpreferred embodiments, the culture medium lacks animal-derivedcomponents. As used herein, “animal-derived” components are anycomponents that are produced in an intact animal (such as, e.g.,proteins isolated and purified from serum), or produced using componentsproduced in an intact animal (such as, e.g., an amino acid made by usingan enzyme isolated and purified from an animal to hydrolyse a plantsource material). By contrast, a protein which has the sequence of ananimal protein (i.e., has a genomic origin in an animal) but which isproduced in vitro in cell culture (such as, e.g., in a recombinant yeastor bacterial cells or in an established continuous eukaryote cell line,recombinant or not), using media lacking components produced in, orisolated and purified from, an intact animal is not an “animal-derived”component. For example, insulin produced in a yeast or a bacterial cell,or insulin produced in an established mammalian cell line, such as,e.g., CHO, BHK or HEK cells, or interferon produced in Namalwa cells,does not constitute an “animal-derived” component.” Accordingly, a cellculture medium lacking animal-derived components is one that may containanimal proteins that are recombinantly produced; such medium, however,does not contain, e.g., animal serum or proteins or other productspurified from animal serum. Such medium may, for example, contain one ormore components derived from plants.

In yet other embodiments of the invention, the medium used during thecell growth phase contains concentrated medium, i.e., medium thatcontains higher concentration of nutrients than is normally necessaryand normally provided to a growing culture. One skilled in the art willrecognize which cell media, inoculation media, etc. is appropriate toculture a particular cell, e.g., animal cells (e.g., CHO cells). Forexample, one of skill in the art will be able to suitably select for aparticular culture the amount of glucose and other nutrients, such asglutamine, iron, trace elements, and the like, as well as other culturevariables, such as, e.g., the amount of foaming, osmolality, etc. (see,e.g., Mather, J. P., et al. (1999) “Culture media, animal cells, largescale production,” Encyclopedia of Bioprocess Technology: Fermentation,Biocatalysis, and Bioseparation, Vol. 2:777-785 (especially pages 780 to783); U.S. Patent Application Publication No. 2006/0121568 (especiallyparagraphs [0144] to [0185] and [0203] to [0331]); both of which arehereby incorporated by reference herein in their entireties). Thepresent invention also contemplates variants of such known media,including, e.g., nutrient-enriched variants of such media, concentratedmedia, chemically-defined media, serum-free media, and media otherwisemodified in accordance with various embodiments of the invention.

The continuous culture system is not limited to any type ofnon-anchorage dependent mammalian cells. The mammalian cells may begenetically modified mammalian cells expressing a recombinantpolypeptide (and/or recombinant virus) of interest, or unmodifiedmammalian cells expressing a polypeptide (and/or virus) of interest. Anumber of mammalian cell lines are suitable host cells for recombinantexpression of polypeptides and/or viruses. Mammalian host cell linesinclude, for example, COS, PER.C6, TM4, VERO, MDCK, BRL-3A, W138, HepG2, MMT, MRC 5, FS4, CHO, 293T, A431, 3T3, CV-1, C3H10T1/2, Colo205,293, HeLa, L cells, BHK, HL-60, FRhL-2, U937, HaK, Jurkat cells, Rat2,BaF3, 32D, FDCP-1, PC12, Mlx, murine myelomas (e.g., SP2/0 and NS0) andC2C12 cells, as well as transformed primate cell lines, hybridomas,normal diploid cells, and cell strains derived from in vitro culture ofprimary tissue and primary explants. Any mammalian cells that can beadapted for suspension culture may be used in the disclosed cell culturemethod. A nonlimiting example includes CHO cells, which are anchoragedependent when cultivated in the presence of serum and suitablesurfaces, but are easily adapted to growth in suspension culture (see,e.g., Rasmussen 1998, Cytotechnology 28: pp 31-42, especially pages34-37, regarding culturing a serum-free CHO cell line). Also anymammalian cell capable of expressing the polypeptide and/or virus ofinterest (whether produced recombinantly or not) may be used in thedisclosed cell culture methods. In one embodiment, the continuous cellculture method disclosed herein may be used to adapt an anchoragedependent cell line to a non-anchorage dependent cell line. Numerouscell lines are available from commercial sources such as the AmericanType Culture Collection (ATCC). In one embodiment, the continuous cellculture system is used to culture genetically modified CHO cells.

As described herein, the continuous cell culture system allows forrecovery of a polypeptide and/or virus of interest (e.g., a recombinantpolypeptide and/or recombinant virus). The polypeptide and/or virustypically is recovered from spent medium that has been removed from thesystem. Advantages of preferred embodiments of the invention includereducing the residence time of the protein and/or virus in thebioreactor, which is particularly useful for products susceptible todegradation. However, the continuous cell culture system as disclosedherein is not limited to such labile polypeptides or viruses, and may beused for the recovery of other polypeptides or viruses of interest.

The present invention relates to methods for the improved large-scalecontinuous culture of mammalian cells that express one or more proteinsand/or viruses of interest, whether from endogenous genes or naturalinfection, or subsequent to introduction into such cells of recombinantgenes encoding the protein or virus of interest. Such proteins include,as nonlimiting examples, enzymes, hormones, antibodies, proteinreceptors, fusion proteins (e.g., fusions of soluble receptors and theFc domain of an IgG), vaccines, cytokines, chemokines, growth factors,blood factor proteins etc. In one embodiment, the protein of interest isADAMTS13. In another embodiment, the protein of interest is Factor VIIor Factor VIII. In another embodiment, the protein of interest isalpha-1-proteinase inhibitor.

The present invention also relates to methods for the improvedlarge-scale continuous culture of mammalian cells that express one ormore viruses of interest (including viral particles and viral vectors),whether wildtype or recombinant. Nonlimiting examples of such wildtypeor recombinant viruses, viral particles and/or viral vectors includeAdenoviruses, Herpes viruses, Retrovirusess, Lentiviruses, Influenzaviruses, etc. Production of recombinant viruses, viral particles, anduse of recombinant viral vectors are well-known in the art. For theexpression of virus, the method according to the invention isparticularly suitable for, but not limited to, infection with andexpression of non-lytic viruses such as e.g. Hepatitis A and TBE in,e.g., Vero cells, or Lentivirus in, e.g., HEK293 or other human ormammalian cell lines.

Once the medium has been removed from the culture unit, it may besubjected to one or more processing steps to obtain the protein and/orvirus of interest. Downstream processing steps include, withoutlimitation, centrifugation and/or filtration to remove cells notpreviously withdrawn from the culture; affinity chromatography,hydrophobic interaction chromatography; ion-exchange chromatography;size exclusion chromatography; electrophoretic procedures (e.g.,preparative isoelectric focusing (IEF), differential solubility (e.g.,ammonium sulfate precipitation), extraction, and the like. See,generally, Scopes, Protein Purification, Springer-Verlag, New York,1982; and Protein Purification, J.-C. Janson and Lars Ryden, editors,VCH Publishers, New York, 1989.

In certain embodiments, the mammalian cells are subjected to apre-culturing step, e.g., a pre-culturing step of adapting the cells forproduction of the polypeptide and/or virus of interest. In someembodiments, the adapting comprises pre-culturing the cells insuspension, e.g., for a time to allow the culture to reached a desiredfinal working volume, e.g., of about 5 L, about 8 L, about 10 L, about12 L, about 15 L, or about 20 L, etc. At this point, the culture can beswitched to a continuous medium feed and operated according to thecontinuous cell culture system described herein.

Exemplary embodiments of the invention disclosed herein are furtherdiscussed in the Examples provided below. The Examples, however, andtheir particular details, are in no way intended to limit the invention.

EXAMPLES Example 1

Chemostat cultures were prepared with a recombinant CHO cell lineexpressing human ADAMTS13 in chemically defined BACD-A13 medium(enriched DMEM/F12 formulation), which is supplemented as shown in Table1.1.

TABLE 1.1 Composition of cell culture medium BACD-A13 ComponentConcentration [g/kg] DMEM/HAMS F12 BaxS9 12.74 L-Glutamine 1.3Synperonic 1.00 Ethanolamin 0.00153 ZnSO4•7H2O 0.001 NaHCO3 1.5

Recombinant CHO cells expressing human ADAMTS13 were adapted to achemically-defined medium, namely BCS medium, as shown in Table 1.2.

TABLE 1.2 Composition of cell culture medium BCS Component Concentration[g/kg] DMEM/HAMS F12 Bax Special 11.75 L-Glutamin 0.9 Synperonic 1.00Ethanolamin 0.00153 Putrescine•2HCl 0.0036 FeSO4•7H2O 0.0006 NaHCO3 2.0

A Development Working Cell Bank was thawed and cell inoculum wasprepared in BCS medium. Cells were transferred to a 10 L culture unitwith Rushton type impellers and cultivated in repeated batch cultures inBACD-A13 medium with inline controlled parameters, as follows: pH 7.10,temperature 36° C., and DO 20% air saturation.

After 2 batch cycles, cultures reached a final working volume of 10 L,and the culture was switched to continuous medium feed on day 4 andoperated until day 18 in a chemostat mode.

From this culture, a second 10 L culture unit with Rushton type impellerand a cell retention device was inoculated for chemostat-like perfusion,using the same cell culture medium, and cultivated for 8 days inrepeated batch culture. CYTOPORE 2™ Microcarriers (GE Healthcare) wereadded (0.25 g/L), and the culture unit was further operated incontinuous chemostat-like perfusion mode in parallel to the otherculture unit in chemostat mode. Culture units were agitated with Rushtontype impellers with baffles at 140 rpm, which corresponds to a specificpower/volume input of approximately 40 W/m³.

Both culture units were operated in parallel for 24 days. Data werecalculated in 3 weekly intervals and an additional 3 day interval (Table1.3. (Chemostat) and Table 1.4 (Chemostat-like perfusion). Data fromfour intervals of Table 1.3. (days 26-32; days 33-39; days 40-46; anddays 47-49) were directly compared with the four intervals shown inTable 1.4 (days 9-15; days 16-22; days 23-29; and days 30-32).

Samples from the culture units were taken and analysed for ADAMTS13concentration by ELISA, and ADAMTS13 activity by FRETS-73 assay. Cellcounts were determined by Nucleocounter technology. For perfusioncultures, total cell counts and cell counts in the supernatant weremeasured separately to calculate the relative cell retention. Dilutionrates were measured and used for calculating growth rates and volumetricproductivities. The equations are provided below

Growth rate (μ) in the chemostat culture was calculated using theequation: μ=D+ln(X_(t)/X_(t-1))/(t−t⁻¹), wherein D=Dilution rate,X_(t)=Total cell density at time t, and (t−t⁻¹)=Time between t and t⁻¹.

Growth rate (μ) in the chemostat-like perfusion culture was calculatedusing the equation μ=ln(X_(t)/X_(t-1))/(t−t⁻¹)+D×(log mean X_(SN)/logmean X)/(t−t⁻¹); wherein D=Dilution rate, X_(t)=Total cell density attime t, (t−t⁻¹)=Time between t and t⁻¹, log mean X=logarithmic mean oftotal cell density=(X_(t)−X_(t-1))/(ln(X_(t))−ln(X_(t-1))), and log meanX_(SN)=logarithmic mean of cell density supernatant.

Cell retention rate was calculated as =100×(1−X_(SN)/X)[%]

TABLE 1.3 Fermentation Suspension Data for Chemostat Culture AdamtsAdamts spec. P Interval ZZ.-Nuc. D μ Frets ELISA Activity P Frets ELISAqp Frest qp ELISA Days [1 E6/ml] [1/d] [1/d] [mU/ml] [μg/ml] U/mg[U/L/d] [mg/L/d] [mU/E06/d] [μg/E06/d] 0-4 n.a. Batch 0.420 n.a. n.a.n.a. n.a. n.a. n.a. n.a.  5-11 2.76 0.353 0.369 4314.4 4.09 1055 15231.44 551 0.52 12-18 2.01 0.309 0.299 9148.8 8.45 1083 2830 2.61 14071.30 19-25 1.82 0.311 0.300 8609.6 8.67 993 2677 2.70 1473 1.48 26-321.76 0.309 0.304 8868.2 9.96 891 2742 3.08 1560 1.75 33-39 1.84 0.3210.337 7845.4 10.10 777 2519 3.24 1369 1.76 40-46 1.97 0.330 0.340 8098.79.24 877 2674 3.05 1358 1.55 47-49 2.02 0.361 0.343 7353.7 9.30 791 26583.36 1314 1.66 Mean 1.90 0.330 0.331 8041.5 9.65 834 2648 3.18 1400 1.6826-49

TABLE 1.4 Fermentation Suspension Data for Chemostat-like culture TotalCC Cell Adamts Adamts spec. nterval CC Supernatant. retention D μ FretsELISA Activity P Frets P ELISA In qp Frets In qp ELISA Days [1 × 10E6/ml] [1 × 10 E6/ml] % [1/d] [1/d] [mU/ml] [μg/ml] U/mg [U/L/d][mg/L/d] [mU/E06/d] [mU/E06/d] 0-8 n.a. n.a. n.a. Batch 0.398 n.a. n.a.n.a. n.a. n.a. n.a. n.a.  9-15 2.04 0.82 60 0.37 0.271 5804.4 6.25 9292176 2.34 1068 1.15 16-22 3.75 1.14 70 0.44 0.192 8863.5 10.18 871 38894.46 1037 1.19 23-29 5.86 1.53 74 0.53 0.178 11873.6 11.03 1076 62925.84 1073 1.00 30-32 8.05 1.30 84 0.62 0.175 9398.6 10.61 886 5794 6.54719 0.81 Mean 4.93 1.20 72 0.49 0.204 8985.0 9.52 941 4538 4.80 974 1.049-32

The specific growth rate of the cells in batch culture was about 0.42d⁻¹. After switching to chemostat culture, a decline of the specificgrowth rate to approximately 0.30 d⁻¹ was observed, which is indicativefor growth limiting conditions in the chemically-defined medium. Celldensities equilibrated in the range of 1.8-2×10⁶ cells/mL. Under suchcontinuous culture conditions, the steady state was reached (beginningin the interval 12-18), and over 2500 U/L/d can be achieved.

As noted above, after inoculating the culture unit with a cell retentiondevice, on day 18, cells were further cultivated in a continuouschemostat-like perfusion mode. Due to the cell retention, cell densityin the chemostat-like perfusion cultures were increased. However due tothe relatively low cell retention rate (mean 72%; range 60%-84%) thecell densities were maintained at a relatively low level of <1×10⁷cells/mL (mean 4.93×10⁶ cells/mL) and the maximum dilution rate was 0.62

The low cell retention rate also allowed the cells to maintain acontinuous specific growth rate of 0.18-0.27 d⁻¹ without leading toexcessive cell densities. This is considered an advantage forrecombinant cells, which express recombinant proteins in a growthassociated manner. Despite a reduced specific productivity (e.g. 974mU/E06/d vs. 1400 mU/E06/d) the volumetric productivity was increased byover 70% from 2648 U/L/d to 4538 U/L/d due to the approximately 2.8 foldhigher cell density. The specific activity of the recombinant proteinalso improved (941 U/mg vs. 834 U/mg), probably due to the reducedresidence time in the culture unit, thereby improving stability, or anyother beneficial effect on the structure and function of the expressedrecombinant ADAMTS13.

Example 2

Chemostat cultures were prepared with a recombinant CHO cell lineexpressing human ADAMTS13 in chemically-defined BACD-A13 medium(enriched DMEM/F12 formulation), which is supplemented as shown in Table1.1.

Recombinant CHO cells expressing human ADAMTS13 were adapted to achemically-defined medium, namely BCS medium, as shown in Table 1.2. ADevelopment Working Cell Bank was thawed and cell inoculum was preparedin BCS medium. Cells were transferred to a 1.5 L culture unit with bladeimpellers and cultivated in repeated batch cultures in BACD-A13 mediumwith inline controlled pH 7.1, temperature 36° C., and DO 20% airsaturation.

After 2 batch cycles, cultures reached a final working volume of 1.5 L,and the culture was switched to continuous medium feed on day 5 andoperated until day 7 in a chemostat mode.

From this culture a second culture unit comprising a cell retentiondevice with blade impellers was inoculated using the same cell culturemedium and cultivated for 1 day in batch culture. CYTOPORE 2™Microcarriers (GE Healthcare) were added (0.25 g/L), and the cultureunit was further operated in continuous chemostat-like perfusion mode inparallel to the other culture unit in chemostat mode.

Both culture units were operated in parallel for 28 days. Data werecalculated in 4 weekly intervals (Table 2.1 (chemostat) and 2.2(chemostat-like perfusion mode)).

Samples from the culture units were taken and analysed for ADAMTS13concentration by ELISA and ADAMTS13 activity by FRETS-73 assay. Cellcounts were determined by Nucleocounter technology. For chemostat-likeperfusion cultures, total cell counts and cell counts in the supernatantwere measured separately to calculate the relative cell retention.Dilution rates were measured and used for calculation of growth ratesand volumetric productivities.

Growth rate (μ) in the chemostat culture was calculated using theequation: μ=D+ln(X_(t)/X_(t-1))/(t−t⁻¹), wherein D=Dilution rate,X_(t)=Total cell density at time t, and (t−t⁻¹)=Time between t and t⁻¹.

Growth rate (μ) in the chemostat like perfusion culture was calculatedusing the equation μ=ln(X_(t)/X_(t-1))/(t−t⁻¹)+D×(log mean X_(SN)/logmean X)/(t−t⁻¹); wherein D=Dilution rate, X_(t)=Total cell density attime t, (t−t⁻¹)=Time between t and t⁻¹, log mean X=logarithmic mean oftotal cell density=(X_(t)−X_(t-1))/(ln(X_(t))−ln (X_(t-1))), and logmean X_(SN)=logarithmic mean of cell density supernatant.

Cell retention rate was calculated as =100×(1−X_(SN)/X)[%]

TABLE 2.1 Fermentation Suspension Data for Chemostat Culture AdamtsAdamts spec. P Interval Total CC D μ Frets ELISA Activity P Frets ELISAqp Frests qp ELISA Days [1 × 10 E6/ml] [1/d] [1/d] [mU/ml] [μg/ml] U/mg[U/L/d] [mg/L/d] [mU/E06/d] [μg/E06/d] 0-5 n.a. Batch 0.507 n.a. n.a.n.a. n.a. n.a. n.a. n.a.  9-15 1.87 0.345 0.340 2879.8 2.987 964 994.01.031 533.0 0.553 16-22 1.21 0.298 0.253 3483.7 3.755 926 1042.9 1.123867.7 0.933 23-29 1.02 0.270 0.262 3726.6 3.928 946 951.8 1.008 957.91.021 30-36 0.83 0.279 0.268 2551.1 2.858 895 691.8 0.780 837.7 0.948Mean 1.23 0.298 0.281 3160.3 3.382 933 920.1 0.986 799.1 0.864 9-36

TABLE 2.2 Fermentation Suspension Data for Chemostat-like PerfusionCulture CC Cell Adamts Adamts Spec. z. P Interval Total CC Supernatant.retention μ Frets ELISA Activity P Frets ELISA q Frests q ELISA Days [1× 10 E6/ml] [1 × 10 E6/ml] % D [1/d] [1/d] [mU/ml] [μg/ml] U/mg [U/L/d][mg/L/d] [mU/E06/d] [μg/E06/d] 02-08 1.96 1.22 38 0.449 0.262 2767.43.180 870 1241.6 1.427 634.6 0.729 09-15 2.95 1.12 62 0.442 0.256 4726.54.788 987 2090.0 2.117 707.7 0.717 16-22 3.90 1.06 73 0.592 0.207 3855.34.071 947 2281.5 2.409 585.6 0.618 23-29 3.57 1.00 72 0.604 0.205 2671.23.027 882 1613.4 1.829 451.8 0.512 30-36 3.52 1.13 68 0.594 0.222 2559.42.761 927 1519.5 1.639 431.3 0.465 37-43 4.20 1.18 72 0.571 0.219 2828.72.905 974 1615.2 1.659 384.7 0.395 44-49 4.30 1.21 72 0.566 0.159 2705.03.110 870 1530.0 1.759 355.9 0.409 Mean 3.09 1.10 61 0.522 0.232 3505.13.767 922 1806.6 1.945 594.9 0.644 02-29 Mean 3.49 1.13 68 0.545 0.2193159.1 3.406 922 1698.8 1.834 507.3 0.549 02-49

The specific growth rate of the recombinant CHO cells expressingADAMTS-13 in batch culture was about 0.51 d⁻¹. After switching tochemostat culture, a decline in the specific growth rate to less than0.30 d⁻¹ was observed, which is indicative of growth limiting conditionsin the chemically-defined medium. Cell densities equilibrated in therange of 0.8-1.2×10⁶ cells/mL with a specific growth rate of 0.25-0.27d⁻¹. Under such continuous culture conditions, the steady state wasreached (beginning in the interval 16-36). The mean productivity of all4 intervals from day 09-36 was 920 U/L/d.

As described above, after inoculating the culture unit with a cellretention device on day 7, cells were further cultivated in continuouschemostat-like mode. Data from four intervals (days 9-15; days 16-22;days 23-29; and days 30-36) and the mean value of days 9-36 of Table 2.1were directly compared with the data from four intervals (days 2-8; days9-15; days 16-22; and days 23-29) and the mean value of days 2-29 ofTable 2.2.

The chemostat-like perfusion culture was then operated (without achemostat as reference) for another 3 intervals (days 30-36; days 37-43;and days 44-49) to demonstrate the long term stability of thechemostat-like perfusion culture. Mean values of 7 weeks of continuousculture are provided in Table 2.2 (mean of days 2-49).

Due to the cell retention, cell density in the perfusion culturesincreased. However due to the relatively low cell retention rate (meanof days 02-29: 61%, mean of days 02-49: 68%. range 38%-72%) the celldensities were maintained at the relatively low level of <5×10⁶ cells/mL(mean of days 02-49=3.49×10⁶ cells/mL). The maximum dilution rate ofapproximately 0.60 d⁻¹ was reached in the 3rd interval and then remainedconstant between 0.55 d⁻¹-0.60 d⁻¹ until the end of the experiment (meanof days 02-49: 0.55 d⁻¹).

The low cell retention rate also allowed the cells to maintain acontinuous specific growth rate of 0.16-0.2 d⁻¹ (mean of days 02-49:0.22 d⁻¹) without leading to excessive cell densities. Despite a reducedspecific productivity (e.g. 595 mU/E06/d vs. 799 mU/E06/d) thevolumetric productivity was increased by over 90% from 920 U/L/d to 1807U/L/d (mean of first 4 intervals) due to the approximately 2.8× highercell density. The productivity of the chemostat-like culture remainsrelatively stable in the range from 1500-1600 U/L/d for the additional 3intervals.

As in Example 1, it was demonstrated that this approach of stabilizingcontinuous suspension cultures with a lowered cell retention rate cancompensate for the disadvantages of severe growth limitations undercertain culture conditions (e.g., when using chemically-defined media).Accordingly, controlling cell retention allows the culture to bemaintained at relatively moderate cell densities and relatively low andconstant dilution rates. Also due to the relatively low cell retentionrate, the culture retained more of the characteristics of a continuoussuspension culture, as shown, e.g., by the specific growth rates.

All patents, patent publications, and other publications referred toherein are hereby incorporated by reference, to the same extent as ifeach individual publication, patent, or patent publication wasspecifically and individually indicated to be incorporated by reference.

Certain modifications and improvements will occur to those skilled inthe art upon a reading of the foregoing description. It should beunderstood that all such modifications and improvements have beendeleted herein for the sake of conciseness and readability. Nonetheless,all such modifications and improvements are contemplated as within thescope of the instant invention and are properly within the scope of thefollowing claims.

1. A method for producing a polypeptide and/or virus of interest in acontinuous cell culture, said method comprising (a) culturing mammaliancells expressing the polypeptide and/or virus of interest in acontinuous cell culture system, wherein said cell culture systemcomprises a cell retention device and has a dilution rate (D) of lessthan 2 d⁻¹ and a cell density of less than 2×10⁷ cell/mL; and (b)recovering said polypeptide and/or virus of interest from medium removedfrom said cell culture system.
 2. The method of claim 1, wherein saidcell retention device produces a cell retention rate of less than 90%.3. The method of any of the above claims, wherein said dilution rate isbetween 0.1 and 1.0 d⁻¹.
 4. The method of any of the above claims,wherein said cell density is less than 1×10⁷ cell/mL.
 5. The method ofany of the above claims, wherein said cell culture system has a ratio ofthe dilution rate and specific growth rate (D/μ) between 1.2 and
 5. 6.The method of any of the above claims, wherein said cell culture systemhas a specific growth rate of between 0.2 d⁻¹ and 0.8 d⁻¹.
 7. The methodof any of the above claims, wherein said cells are cultured in said cellculture system for more than 20 days.
 8. The method of any of the aboveclaims, wherein said cells are cultured in said cell culture system formore than 40 days.
 9. The method of any of the above claims, whereinsaid cells are cultured in said cell culture system for more than 50days.
 10. The method of any of the above claims, wherein said dilutionrate and said cell density are maintained for at least 80% of the timethe cells are cultured in said cell culture system.
 11. The method ofany of the above claims, wherein said cell retention device comprises amacroporous microcarrier.
 12. The method of any of the above claims,wherein said cells are cultured in a serum-free medium.
 13. The methodof any of the above claims, wherein said cells are cultured in at least250 L of medium.
 14. The method of any of the above claims, wherein saidcells are non-anchorage dependent cells.
 15. The method of any of theabove claims, further comprising, before the culturing step,pre-culturing the cells in suspension.
 16. The method of any of theabove claims, wherein said cells are genetically modified to expresssaid polypeptide and/or virus of interest.
 17. The method of claim 16,wherein said cells are CHO cells.
 18. The method of claim 16, whereinsaid polypeptide is a disintegrin-like and metallopeptidase withthrombospondin type 1 motif 13 (ADAMTS13) protein.
 19. A compositioncomprising a polypeptide and/or virus of interest produced according tothe method of any of the above claims.
 20. A composition comprising arecombinant ADAMTS13 protein produced according to the method of any ofthe above claims.